940 
Lee . — Notes on the Anatomy and Morphology of 
Stated briefly, the question resolves itself into whether the medullary 
bundles are a primary character, or whether they have been acquired 
later owing to the necessity of some means of conduction in the water- 
storing pith. 
In this connexion it is interesting to note that Scott and Brebner, in 
a paper on internal phloem, say : ‘It is probable also, that the pith-cells 
themselves may be able to discharge both storing and conducting functions 
more efficiently when brought into direct relation with the phloem and its 
proteid contents.’ 1 In Pachypodium the efficiency of the pith as a water- 
storing tissue is much increased by the presence of medullary bundles. 
Correlated with the presence of storage parenchyma and of medullary 
bundles is the small quantity of wood, in which Pachypodium differs from 
most Apocynaceous plants. Another adaptation to the habitat is seen 
in the possible water-storing and protective functions of the spines. In 
other characters, e. g. the occurrence of calcium oxalate and laticiferous 
elements, and the superficial origin of the cork, Pachypodium agrees with 
the other members of the order. 
Regarding the origin of the medullary bundles all that can be said is 
that they have been seen to branch off from the ring bundles, and have 
no definite orientation. A study of the seedling structure would be 
necessary to follow up their development. In other genera of Apocynaceae 2 
which possess medullary bundles there is present a cambium outside the 
inner phloem which gives rise to xylem, so that inversely orientated medul- 
lary bundles are formed. No sign of such a cambium has been seen in 
P achypodium^ where the medullary bundles are simply branches of the ring 
bundles. 
In conclusion, I wish to express my thanks to Dr. Pearson for advice 
and many helpful suggestions in this investigation ; and also to Miss E. L. 
Stephens for her kind assistance in preparing it for the press. 
6. Summary. 
i. The stem is fleshy and consists to a great extent of the pith. 
%. Branching and anastomosing medullary bundles varying in size and 
orientation traverse the pith in all directions. 
3. The bundles of the ring are collateral, separated by wide medullary 
rays, and have very little secondary tissue. 
4. Apparently no intraxylary phloem bundles are present. It is 
suggested that they are present, but possess xylem, and so form the network 
of medullary bundles. 
5. Laticiferous cells are present in the cortex and pith of the stem, the 
1 Scott and Brebner ( 4 ), p. 261. 2 Solereder ( 5 ), p. 531. 
